In general, conventional handheld digital cameras are equipped with an illumination device, such as a flash, for illuminating a scene during image capture. In some cases, the use of a flash results in false conversion of the eye color. For example, the use of a flash device can result in the appearance of red pupils (red-eye) or eyeshine (in many species other than human). Red-eye is typically observed when the flash light occurs too fast for the pupils to close. As a result, a large portion of the light from the flash passes into the eye through the pupil, reflects off the fundus at the back of the eyeball, and exits back out through the pupil before the end of image capture by the camera. The red color of the red-eye effect is due to the color of the fundus, caused by pigmentation in the subject being photographed.
Red-eye generally occurs under certain conditions. In particular, when the angle formed by the flash, the eye, and the lens is smaller than the angle formed by the flash, the back of the eye, and the edge of the pupil, the reflected light can reach the camera. This can occur when the flash is close to the lens, such as in a small handheld camera, and the camera is far enough from the subject. Therefore, as the dimensions of a flash-equipped cameras are reduced, as the proximity of lens and the flash in such cameras further reduces the angle formed by the flash, the eye, and the lens, increasing the chances of red-eye occurring.
There are two conventional approaches for solving the red-eye problem: prevention and correction. Prevention generally involves configuring a camera to fire at least one pre-flash to cause contraction of the pupil, which reduces the angle formed by the flash, the back of eye, and the edge of pupil. This can prevent reflected light from reaching the lens. Correction generally involves automatic or semi-automatic image processing system for modifying the color of the pupil to eliminate the undesired redness. In semi-automatic solutions, a user typically marks the location of the defective pupil color, and the image processing system performs the correction. In fully automatic correction solutions, the image processing system is configured to both detect and correct the undesired redness automatically using digital image processing techniques.